How can I keep bugs out of my room with a window AC unit installed?

How can I COMPLETELY cover any cracks between the window and air unit besides cardboard and tape? I covered up every inch I could see last year and bugs would still get in. Not anything insane, but I HATE ALL bugs... gnats, spiders, moths and stink bugs.. I don't know if they're coming through the actual unit itself (it's pretty new), I think they may just be finding ways around it, but again, I covered every square inch I could see by laying a towel down on the window sill so there wouldn't be any cracks showing, and masked both sides and the top completely with tape so that no bugs would get through.. they still find their way in. Is there a special filter or screen or something I can cover the back of it with? I live in a townhouse where my bedroom is literally probably 85 degrees or more. It's getting unbearable and the only reason I don't want to put the unit in is because of bugs.. I don't know how much more I can take. My dad doesn't believe in turning the central air on. He thinks putting window units in each room and blasting them 24/7 is more cost-efficient than running central air. Don't know if this is true, but seems to be 3 window units in a house would make it worse. Anyway.. How do I stop the bugs??